Einstein’s theories of general and special relativity can be used to actually prove that time travel is possible. Through the phenomenon of time dilation, which states that bodies moving at high velocities experience a time that ticks slower than the time measured at zero velocity. Not as much time elapses for a moving body as does for everything else. Therefore the body travels into the future. Phenomena known as wormholes and closed time like curves are possible means of time travel into the future and the past.

Travelling into the past is a task which is much more difficult than travelling into the future. Its theory involves complicated scenarios of tears in four dimensional space-time, and travelling near the speed of light. Obstacles preventing our attempts to cheat time include our inability to move even close to the speed of light, and finding a source of energy as powerful as an exploding star. Numerous arguments are proposed that that prevent time travel into the past. Both common sense and scientific fact can be used to paint scenarios that become serious obstacles.

Development of spacecraft that can travel at speeds on the order of two hundred million meters per second, at this speed we are near the speed of light, where time actually slows down. This is what’s known as time dilation. Einstein’s theories predict that the faster a spacecraft moves, the slower time ticks inside of it. Imagine that a rocket ship takes off from earth and approaches the speed of light. If we were to watch it from earth with a very powerful telescope as it travelled away from us, we would see everyone inside the ship as being frozen in time. To us their time would slow down, but to them nothing would change! This has been measured in the laboratory and on location using atomic clocks, aircraft, satellites and rockets. It is proven that time slows down the faster you move.

Now that we know that it is possible to travel into the future by moving at great speeds, the next problem is how to travel in time a respectable amount without having to sit in a fast moving spaceship for years. This problem is solved by the theoretical existence of what are know as closed time like curves, and wormholes.

Einstein’s special and general theories of relativity combine three-dimensional space with time to form four dimensional space-time. Space-time consists of points or events that represent a particular place at a particular time. Your entire life thus forms a sort of twisting, turning worm in space time! The tip of the worm’s tail would be your birth and its head is the event of your death. The line which this worm creates with its body is called that object’s world line. Einstein predicts that world lines can be distorted by massive bodies such as black holes. This is essentially the origin of gravity, remember. Now if an object’s world line were to be distorted so much as to form a loop that connected with a point on itself that represented an earlier place and time, you are connected to that earlier place in time. Time like means that the body under consideration experiences time that increases in one direction along its world line. These closed loops are shortcuts through space and time known as wormholes.

Wormholes are holes in the fabric of four dimensional space-time, that are connected, but which originate at different points in space and at different times. They provide a quick path between two different locations in space and time. This is the four dimensional equivalent of pinching two pieces of a folded sheet of paper together to make contact across the gap. Distortions in space cause the points separated by the gap to bulge out and connect. No more problems of travelling in a rocket ship for years to get into the future.

Wormholes and closed time like loops appear to be the main ways that time travel into the past would be possible. The limitation on this time travel into the past is that it would be impossible to travel back to a time before the point at which the loop started. Although the aforementioned theories of general relativity are consistent for closed time like curves and wormholes, the theories say nothing about the actual process of travelling through them. Quantum mechanics can be used to model possible scenarios, and yields the probability of each possible output. Quantum mechanics, when used in the context of time travel, has a so-called many-universe interpretation. This was first proposed by Hugh Everett III in 1957. It encompasses the idea that if something can physically happen, it does in some universe. Everett says that our reality is only one of many equally valid universes. There is a collection of universes, called a multiverse. Every multiverse has copies of every person, structure, and atom. For every possible event, every possible outcome is said to be played out on a different universe. This interpretation of quantum mechanics is quite controversial however, but does elicit the notion that it may be impossible to travel backward in time to our own universe or dimension. One must consider what past would be the destination of a time traveller.

The proposal of time travel is backed by scientific theory, but that is not enough to make it realistically possible. Numerous arguments are proposed that that prevent time travel into the past. Both common sense and scientific fact construct serious obstacles. A major argument against time travel into the past is called the autonomy principle, better know as the grandfather paradox. This paradox is created when a time traveller goes back in time to meet his or her grandfather. Now upon their introduction it would be possible to change the course of events that lead up to your grandfather and grandmother marrying. You could tell him something about a family secret to convince him you are who you say you are, and he may proceed to tell his soon to be wife. She may in turn doubt his sanity and have him committed. Thus your grandparents would never have your mother, and therefore you couldn’t be born! But then how could you have ever existed to travel back in time if you don’t exist? You would have had to have been created via autonomy. The next question would be, if your mother was never born, then when you return to the future would anything you did in your life exist? Or would you, your friends, your home etc. never have existed? This is clearly an inconsistency paradox that would rule out time travel, yet interestingly enough the laws of physics do not forbid such excursions. The multiverse concept eradicates the problem of the autonomy principal, because it allows time travel to the past, but to a different universe. You would meet the person who was your grandfather in your universe, but never married your grandmother in his universe. In the universe that you travelled to, you never existed.

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